This invention relates generally to electrophotographic imaging and more particularly provides a method and apparatus for producing color reproductions from color separated images wherein the color reproductions comprise sequentially applied superimposed toned latent images successively produced on a single electrophotographic member in registry and transferred simultaneously to a receptor in a single step.
The invention herein is applicable generally for forming color reproductions. However, it is useful particularly to produce color proof prints for the printing industry.
Color proofs are employed to enable the printing craftsman to ascertain quickly, efficiently and accurately the results of color separation, the corrections required to be made to the color separation and the suitability of the corrected color separations for platemaking. The ability to simulate and/or to predict the final appearance of the ultimate print copy on the particular receptor medium used for the print run is an important factor in the color printing process.
Separation proofs are made directly of the photoreproduction apparatus to determine the results of the separation process and the identity and character of any corrections needed. Of considerable importance is the capability of accurate and reproducible evaluation of factors such as color balance, tone reproduction, shadow detail, image sharpness, and contrast, among others. Economy and speed in making such proofs are sought after goals in color proofing. Equally important are reliability, reproducibility and predictability. The proof must reproduce the color separation film exactly without distortion or loss. Exact replicas of the printing ink characteristics should be reproduced so that overprinting colors will be the same on the proofs as they are with printing inks employed on the printed sheet.
The pre-press proof is intended to reproduce the result which will be obtained using the printing press, indiciating the effects of the paper surface, ink strength, gloss, etc. The pre-press proof should show the same printing characteristics as the finished printed result.
Several photomechanical processes for prepress-proofing are available. These systems fall into two catagories, namely overlay systems and superimposition systems.
Overlay systems consist of a set of transparent light sensitive films which are dried or pigmented to simulate the four process colors, yellow, cyan, black and magenta. Each screened separation is exposed to the appropriate film and developed chemically. After development, four separate images are produced which are superimposed in register. The result is viewed as a transparency. These are generally employed where a quick and inexpensive proof is required and normally are not a satisfactory match for the printed reproduction. The whites are gray and the result, very glossy, suffering from internal reflections between film layers which generally cause color changes in overprinted colors. They are economical to produce, require no special equipment and are extensively used for internal checking.
Superimposition systems involve the production of an image on an integral backing sheet either specific to the process or of the type on which the final print will be made. These processes include the Cromalin process of DuPont Co., the Transfer Key process of Minnesota Mining and Manufacturing Corporation, the Gevaproof process of Agfa-Gevaert and the Remak process of Chemical Corporation of Australia, Pty. Ltd.
The Cromalin process involves the lamination of a tacky transparent photopolymer film to a base sheet under heat and pressure. The film is hardened by exposure to ultraviolet light. The protective cover sheet is removed and toning powder of the appropriate color is dusted over the surface. The toner adheres only to the areas where no exposure has been received and the polymer remains tacky. The proof is produced by repeating this procedure four times, once for each separation. The base material is a heavy cast coated paper or a boardlike member, thus requiring specially made stock.
The Transfer Key process can employ any base stock. A set of four transparent light sensitive films are applied which have been pigmented to simulate the four process colors. These films are coated with a pressure sensitive adhesive and may be adhered to a base stock to form the laminate. The exposed image is polymerized by exposure to ultraviolet light. The unhardened areas are removed by a solvent with the proof being built up one layer at a time. This process can be improved by producing the layers on a transparent base which in turn is laminated to a base sheet using a spacer to simulate dot gain.
The Gevaproof process also uses laminations to a base stock similar to the Transfer Key process.
The REMAK process is an electrostatic process wherein a sheet of paper coated with a zinc oxide/resin binder composition is charged electrostatically and exposed to light through a color separated transparency. The exposed sheet is immersed in a liquid toner bath and electrophoretically toned. The resulting visible image is transferred to any base stock or, alternatively, the proof may be built up by successive exposures and toning on the original base material. Unfortunately, the zinc oxide photoconductor used with the REMAK process is extremely sensitive to changes in temperature and relative humidity, as well as variations in toner lots.
Many of the problems of prior art proofing methods have been solved by the referenced U.S. Pat. No. 4,358,195 granted Nov. 9, 1982. The referenced patent discloses a method and apparatus which takes advantage of the high speed response of Kuehnle U.S. Pat. No. 4,025,339 electrophotographic member using a flat-bed machine having plural toning stations sequentially arranged linearly along a framework. A color separated transparency was mounted on a copyboard and presented to a charged electrophotographic member. The charged member and the transparency were superposed and exposed to a light source. The carrier for the electrophotographic member was manipulated (pivotally inverted) and presented to a movable toning station. The toned member was again inverted for presentation to a transfer means effective to transfer the toned image to a sheet of print stock. The process could be repeated with different separations and toners with registration being obtained by positioning both color separation and electrophotographic medium with registration means provided.
The referenced patent applications disclose and claim a method and apparatus constituting improvements over the state of the art in respect of producing color proofs, including multiple color proofs, as represented both by the above-identified processes and the referenced patent.
Among the advantages, once mounted, the color separation transparency, the imaging member, or any other process related member need not be touched or manipulated so that the sequence of processing steps was capable of proceeding serially and automatically with a minimization of manually operated steps; daylight operational conditions, improvement in control and fine adjustment of background density and/or fog, on-line cleaning, discharge of any residual charge of the electrophotographic member subsequent to transfer and additionally reduces fabrication cost by substantially eliminating high precision components.
In the course of operating the system of the latest copending referenced patent application, a carriage carrying a platen on which an electrophotographic member having a photoconductive surface facing outward, is translated along a linear path past plural functional stations including a charging station, an imaging or exposure station, a toning station, an image transfer station and a cleaning station sequentially, the path being defined in a single horizontal plane by guide means mounted on a framework within a housing. A copyboard is located within said housing at the imaging station and means are provided for mounting a selected transparency thereupon. A toning module is located within the housing, said toning module including a sump containing liquid toner, a generally planar development electrode and means for flowing liquid toner generally uniformly across the development electrode. The toning module preferably is seated at one level normally and is lifted to a second level to place the development electrode in toning proximity with the photoconductive surface of the electrophotographic member when the carriage carrying same arrives at the toning station. The translation of the carriage begins at a home station, preferably the imaging station, and the carriage is translated past a corona generating device at the charging station for application of an electrostatic charge potential to the photoconductive surface of the electrophotographic member carried thereby. After sufficient charge has been applied to said surface, the carriage is translated to the imaging station where the copyboard is raised to establish an intimate engagement with the charged photoconductive surface and radiant energy from a source thereof located below the copyboard is projected to the photoconductive surface through the transparency. Then the copyboard is lowered. A latent electrostatic charge image thus is formed on said surface. The carriage then is translated to the toning station where the toning module has been lifted. Preferably, the toning module is raised to a level to be intercepted by the carriage, and particularly the platen, carried thereby. Resiliently biased slide means provided on the toning module adjacent the development electrode are intercepted by the entry of the platen into the toning station and forced downward against said bias whereby to establish a predetermined toning gap between the development electrode and the photoconductive surface during the passage of the surface through the toning station, the platen riding on said slide means.
One to three or more passes are made before the carriage leaves the toning station to enter the transfer station. At the transfer station, a receptor sheet, such as printing stock, is mounted on a suitable mounting, wetted with an insulating liquid medium, such as a liquid hydrocarbon, e.g., ISOPAR, and roller means are employed to effect transfer of the image to the receptor surface.
The different color toner liquids are carried individually in separate toner modules (containers) respectively, a different color separation transparency being substituted sequentially with the formation of a toner image from each transparency and transfer of the said toner image immediately after its formation. Only one toner image is formed at a time on the electrophotographic member. This toner image is transferred after formation by bringing the electrophotographic member to a transfer station whereat the toner image carried thereon is transferred to a receptor sheet. The member is returned for recycling employing the next in a series of color separation transparencies. Each toner image is transferred successively after formation in superimposed relationship on earlier toner images of the series one after the other on the receptor sheet in registration.
It would be of considerable advantage to eliminate preparing each separate color image on the electrophotographic member and transferring each sequentially in four separate operations while maintaining the required degree of registration on the receptor to form a composite color image thereon. However, it would be anticipated that considerable difficulties would be encountered in charging and toning of each existing toner layer which in turn, would interfere with the achievement of the desired result.
The extension of multicolor electrophotographic imaging technique to on-site oil well logging, seismic tracing, as well as other areas where a major drawback has been the vast quantity of recording paper generated in such fields, could be expected if the requirement of time and multiple proof copies can be eliminated. If the quantity of paper can be reduced as by a one step transfer process, feasibility of electrophotographic imaging would be enhanced materially.